(1) Field of The Invention
The present invention relates to a servo positioning circuit for controlling the positioning of an object under servo control (hereinafter referred to as a "servo object") to a target position by speed control and position control.
(2) Description of the Related Art
Wide use is being made of servo positioning circuits for the positioning of magnetic heads on tracks of magnetic disk devices. In such servo positioning circuits, there is a desire for reduction of the time for movement.
That is, when the number of tracks to move (amount of movement) is indicated, a main processing unit generates a target speed curve in accordance with the number of tracks to move, drives a voice coil motor by coarse (speed) control, and, when reaching near the target position, switches a switching unit to the position control side and performs fine (position) control on the voice coil motor for positioning at the desired track.
During deceleration, however, the real speed draws a path different from the target speed curve. The real speed increases until it reaches the target speed curve, and then decreases. When, however, the differences (amounts of movement) differ, there will be a difference in the gradient of the curve of acceleration at the time of reaching the target speed curve and the gradient of deceleration of the target speed curve at that time depending on the movement. Further, depending on the magnitude of the real speed at the time of reaching the target speed curve, the amount of overshoot will change according to the difference, resulting in a different path of deceleration of the real speed.
This path of deceleration is characterized by a larger error from the decelerating target speed curve the larger the difference. As a result, it takes a longer time for movement.
Further, the gradient of the curve of the real speed differs depending on the magnitude of the difference. This results in a difference of the angle of entry of the real speed upon switching between coarse/fine control. In the case of the real speed having a small gradient and a small difference, fine control is switched to before the real speed reaches zero, the overshoot becomes large, and the time required for positioning by the fine control becomes longer.
In this way, due to the difference in the path of the real speed caused by the magnitude of the difference, there was the problem that the overshoot was large during positioning and that the time required for positioning became longer. Further, there was the problem that the time required for movement of the same remaining movement increased and the problem that the smaller the difference, the more the time could not be shortened.